Palaeomagnetism of Early Triassic limestones from the Huanan Block, South China: no evidence for separation between the Huanan and Yangtze blocks during the Early Mesozoic

Abstract

SUMMARY There has been extensive debate on the timing of the collision between the Huanan and Yangtze blocks ever since Hsu and co-workers (Hsu et al. 1987, 1988) proposed a Mesozoic collision model for South China tectonics. We report new rock and palaeomagnetic data from an Early Triassic limestone formation from the Huanan Block that help to constrain its tectonic history. The samples are from two localities [Jiupi (JT), and Huangben (LT)] in Lianzhou County, northern Guangdong province. Only thermal demagnetization is eective in decomposing a multicomponent remanent magnetization. The majority of samples show three components: a low-temperature component (A), an intermediate-temperature component (B), and a high-temperature characteristic remanent magnetization (ChRM). The A component has a steep inclination and is probably drilling-induced. The B component clusters around the present geomagnetic field direction and is a Brunhes-age viscous remanence. Fold test results suggest that the JT ChRM is pre-tectonic, and that the LT ChRM is a syn- or prefolding remagnetization. Samples from the two localities show quite similar acquisition and thermal demagnetization of the isothermal remanent magnetization (IRM), and suggest that magnetite carries the ChRM component. However, partial anhysteretic remanent magnetization (pARM) spectra, anisotropy of anhysteretic remanence (AAR), rock fabric, and morphology of magnetic extracts are quite dierent for the two localities. The AAR of JT samples shows low anisotropy (average 2 per cent), and reflects a composite compaction and strain fabric. The AAR of LT samples shows a high degree of anisotropy (average 10 per cent), and is clearly consistent with rock deformation fabrics. The magnetic extracts of LT samples are dominated by spherical, botryoidal Ti-poor magnetite, which is probably precipitated from tectonic fluids during deformation, while the JT magnetic extracts are dominated by rod-shaped Ti-poor magnetite, probably derived from the erosion of igneous rocks, although minor amounts of botryoidal magnetite and rod-like magnetite particles can be seen in JT and LT extracts, respectively. The rock fabric and magnetic fabric data, and SEM observations of magnetic extracts corroborate the results of the fold test. The best clustered LT ChRM direction is D=294°, I=12° (a 95 =12°); its pole position is 187°E, 19° S( dp=6°, dm=12°). The assumption of a reversed polarity for the JT ChRM direction (D=198°, I=20°, a 95 =9°) implies that the Huanan Block was in the northern hemisphere during the Early Triassic, and its pole position, 239°E, 68° N( dp=5°, dm=10°) is close to Early Triassic pole positions from the Yangtze Block. A normal polarity for the JT-site ChRM would require tectonic movements inconsistent with the local geology. Therefore, the Huanan and Yangtze blocks were not separated in the Early Triassic. The rotation of the LT locality may have been caused by the collision of the southeast China coastal terranes and/or subduction of the Pacific plate in the Late Mesozoic.